Existing wireless communications networks, such as a wireless Local Area Networks (LANs), contain a multitude of wireless communication devices (e.g., cellular telephones, personal digital assistants, laptop computers) located within a relatively small geographical area and that simultaneously communicate with the same wireless access point. The devices operate on Radio Frequency (RF) channels, the physical resources over which information is passed between the devices. Generally, an RF signal of suitable frequency is modulated by in-phase (I) and quadrature-phase (Q) signals of the same frequency but that are 90° out of phase with each other, thereby generating modulated RF signals. The modulated RF signals are individually amplified, combined and transmitted to a radio receiver on a selected RF channel after having been up-converted to the desired transmission frequency.
When this summed RF signal is received by a receiver, it is separated into its I and Q components, which are then processed by the receiver. This processing may include down-converting the I and Q components with a local oscillator (LO) signal to produce an output signal which is subsequently digitally sampled by an Analog-to-Digital Converter (ADC). The receiver is a Direct Conversion Receiver (DCR) if the LO frequency is approximately equal to the frequency of the received RF signal, and the output signal produced by mixing the RF and the LO signals is a baseband signal (approximately 0 Hz). The receiver is a dual conversion receiver if the LO is offset from the desired received RF signal by an offset frequency typically referred to as an Intermediate Frequency (IF). Dual conversion receivers incorporate additional IF stages to process the IF signal prior to being digitally sampled by the ADC.
Direct conversion receiver (DCR) architectures are generally desirable because they eliminate additional components of intermediate frequency stages, reducing the complexity and cost of the receiver. However, DCRs, especially wideband DCRs, also suffer from problems that are more easily mitigated by using other types of receivers. For example, although it is desired to provide equal individual gains and a 90° phase shift between the I and Q components, in practice, gain imbalance and phase errors exist due to differences between the electronic components in the different signal paths (e.g., filters with mismatched frequency responses) as well as phase errors in due to an imperfectly balanced local oscillator. Phase imbalances lead to the creation of a sideband. These sidebands are lower level images or replicas of the desired signal that are mirrored around the 0 Hz component of the baseband signal. These sidebands are unwanted signals that create an error floor that reduces the bit error rate (BER) performance of the receiver. Moreover, the imbalances may increase in subsequent amplifiers and filters as such the response of these components vary with frequency.
The existence of a substantial sideband thus leads to a significant degradation in system performance. To combat this, countermeasures such as complicated algorithms have been taken in DCRs by attempting to balance the gains and reduce the phase offset. To date, however, sideband suppression in DCRs remains problematic; in some cases being less than 20 dB for a phase error of a few degrees while 60-80 dB is desired. It is thus desirable to improve sideband suppression in DCRs.